Hello and hope you are doing well.
Inability to sleep could be problems with initiating sleep or maintaining sleep is called insomnia. The first step would be to rule the causes for insomnia like sleep disorders. Next it is treated with both medical and non medical therapy. Medical (sedatives) are given in the temporary initial phase, but as he is sensitive to sedatives, he can avoid the same. Non medical measures like sleep hygiene, relaxation therapy, stimulus control, and sleep restriction which are referred to as cognitive behavioral therapies can help restore normal sleep. So, I would advise him to consult your primary care physician for further evaluation. If needed he may refer to a sleep specialist.
Hope this helped and do keep us posted.